Ultrasound (US) has been used for many medical applications, and is generally known as cyclic sound pressure with a frequency greater than the upper limit of human hearing. The production of ultrasound is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. For example, the reflection signature can reveal details about the inner structure of the medium. A well known application of this technique is its use in sonography to produce a picture of a fetus in a womb. There are other applications which may provide therapeutic effects, such as lithotripsy for ablation of kidney stones or high-intensity focused ultrasound for thermal ablation of brain tumors.
A benefit of ultrasound therapy is its non-invasive nature. For example, methods for modulating neural activity include both invasive and non-invasive techniques. Neuromodulation techniques such as deep brain stimulation (DBS) and repetitive transcranial magnetic stimulation have gained attention due to their therapeutic utility in the management of numerous neurological/psychiatric diseases. These methods for stimulating neuronal circuits have been demonstrated to hold promise for the treatment of such diseases and disorders as Parkinson's, Alzheimer's, coma, epilepsy, stroke, depression, schizophrenia, addiction, neurogenic pain, cognitive/memory dysfunction, and others. In the laboratory setting, recent work demonstrated efficacy for millisecond optical control of individual neurons and synapses in intact brain circuits.
The current goals of neurostimulation techniques are to modulate neuronal activity and thereby nervous system function by delivering exogenous energy to intact circuits. However, many of these techniques, such as DBS and vagus nerve stimulation (VNS) require the surgical implantation of stimulating electrodes, an invasive, expensive and even dangerous procedure. For example, the surgical implantation of stimulating electrodes increases secondary medical risks such as infection. The primary cost associated with the surgical implantation of neurostimulation devices is approximately $17,000 to $60,000 per patient, which costs do not take into account the significant costs of pre- and post-operative care.
Ultrasound refers to cyclical vibrations in a frequency range above human hearing, i.e., above about 20 thousand cycles per second (kilohertz, kHz) and including vibrational frequencies of tens and hundreds of millions of cycles per second (MegaHertz, MHz), e.g., a range from about 0.02 to 200 MHz. Ultrasound was first shown to be capable of modulating neuronal activity by inducing reversible suppression. It was earlier demonstrated that ultrasound delivered to the lateral geniculate nucleus of cats in vivo, reversibly suppressed light-evoked potentials in the visual cortex.
Approaches to affecting neural activity in the brain using ultrasound have employed ultrasound frequencies above about 0.6 MHz applied for extended periods of times (several seconds to several minutes), and at intensity levels above about 10 Watts per square centimeter (mW/cm2, where 1 mW=10−3 Watts, and 1 cm=10−2 meters). Many of these approaches are intended to produce macroscopic effects, such as tissue ablation during high intensity focused ultrasound (HIFU). Ultrasound frequencies used for imaging typically range from 2.5 to 7.5 MHz.
What are needed are non-invasive and effective therapies for modulating cellular activity, including the activity of neural cells and other types of cells.